The present invention relates to an improved CCD (charge-coupled device) sensor, in particular, to a CCD sensor suitable for use in high definition television.
The proposed standard for high definition television lays down the following criteria.
______________________________________ Line standard 1125 Field rate 60 Hz, interlaced Aspect ratio 16 .times. 9 (1.77:1) Active number of lines 1035 Active samples per line 1920 ______________________________________
From these criteria, it is possible to derive a specification for a CCD line array sensor suitable for use in telecine equipment for producing high definition television pictures.
This specification is dependent, firstly, on the types of film which are to be converted into video signals using the CCD sensor. In general, there are three types, as follows:
(1) General release, wide screen print which is non-anamorphic with an aspect ratio of 1.85:1;
(2) theatrical release prints with an anamorphic image having an aspect ratio of 2.35:1; and
(3) documentary-type prints having an aspect ratio of 1.37:1
Where, for example, a theatrical release print with an aspect ratio of 2.35:1 is to be displayed on a high definition display format, steps must be taken to enable the whole area of the image to be monitored so that action taking place at the edges of the film image can be displayed if required. FIG. 1 of the drawings shows the theatrical release anamorphic format 2 with the high definition display format 3 in such a way that the high definition display 3 covers the entire height of the anamorphic format 2. To maintain the ability to cover action occuring at the edges of the film image, the entire film image must be scanned and means provided to allow the area of the image to appear in the high definition format to be selected as required. This technique is known as electronic panning.
To maintain the full resolution of 1920 samples laid down in the proposed standard for high-definition television, the film must be scanned line-by-line using a linear sensor array having a number N of active pixels or photo-detectors given by ##EQU1## Thus, the minimum number of photo detectors which could be incorporated into a CCD sensor suitable for use in producing high definition television signals from all three film formats in general use is 2538.
In non-anamorphic wide screen film prints there is a large unused area between neighbouring frames. This is due to the aspect ratio which, as stated above, is 1.85:1. Typically, the frame spacing is 19 mm with the vertical information occupying only about 11.2 mm. This format will, therefore, generate the fastest clock rate requirement of a sensor capable of use with all three film formats.
For a film speed of 24 frames per second, the time T available for the active portion of each frame to be scanned by the sensor is given by ##EQU2##
Since the proposed standard for high definition television signals requires 1035 active lines, the time t available for scanning each line is ##EQU3##
However, the sensor also needs time to allow for video blanking, clamping circuits and photosite transfer time. It is reasonable to allow a total of 3 ms for these requirements, so the maximum time availabe for scanning each line is 20.73 ms. Consequently, a CCD sensor capable of producing high definition television signals from all three film formats in general use must be able to transport at least 2538 charges packets in no more than 20.53 ms. Thus, a clock speed of around 122 MHz is needed.
Current CCD sensor technology of the type used in telecine equipment is illustrated schematicaly in FIG. 2. The sensor 10 includes a plurality of photosites 12 which discharge into two transport registers 14 and 16. Alternate ones of the photosites 12 discharge into each of the two transport registers 14 and 16. The registers operate at around 15 MHz due to limitations imposed by the output detection and amplifier stages.
Clearly, such a sensor cannot meet the requirements of the high definition sensor proposed above. One proposal for overcoming this problem is shown in FIG. 3. In this device each of the two transport registers is divided into two separate registers 14a and 14b and 16a and 16b. The registers of each pair are clocked in parallel but in opposite directions, thus halving the time needed to extract the data from the transport registers. To obtain the required 122 MHz clock rate, each of the four registers 14a, 14b, 16a and 16b would need to be clocked at 30.5 MHz. This clock rate is still high and the device has the additional disadvantage that half the video signal from the transport registers is reverse and requires further manipulation to provide a final output signal.